Our work focuses on T regulatory (Treg) cells, which inhibit both self and allogeneic immune responses making these cells attractive for novel cell-based therapy approaches to promote tolerance and control autoimmune diseases, including type 1 diabetes. However, the use of these cells for therapy is hindered by the inability to generate the sufficient number of cells required to inhibit the desired immune response(s) and achieve stable engraftment of the donor Treg cell inoculums. Furthermore, the constant production of Treg cells from the thymus creates a continuous, competitive barrier that our laboratory has demonstrated must be overcome in order to successfully use Treg cells for therapy. Previous studies from our laboratory demonstrate that the single adoptive transfer of Treg cells into neonatal IL-2R-/- mice, which have impaired IL-2 signaling and Treg cell function, fully prevents severe autoimmunity that would otherwise be lethal for these mice in a matter of a few weeks. Therapy with wild-type Treg cells allows these mice to remain autoimmune-free and to have a normal life span, and is accompanied by life-long donor Treg cell engraftment. Importantly, we show that allo-Treg cells also engraft which confers tolerance to skin allografts and restores self-tolerance. These findings show that under the right biological conditions adoptive transfer of even a small, unmanipulated number of Treg cells is very effective at achieving tolerance and results in long-term engraftment. This IL-2R-/- model offers some clues to the mechanisms that may be required for successful therapy. The overall goals of this proposal are: 1) to elucidate the mechanisms that regulate successful Treg therapy and 2) to develop clinically translatable protocols for the treatment of T1D, using preclinical models to investigate if clinically applicable Treg therapy can be achieved for the prevention or reversal of autoimmune diabetes or to suppress graft rejection of islet cell transplantation for the treatment of diabetes. The present research proposal is designed to test the hypothesis that stable donor Treg engraftment requires immunomodulation and that there is in vivo biological selection by antigen for therapeutic Tregs leading to successful immunotherapy. Based on this hypothesis, the specific aims of this proposal are: 1) to identify mechanisms that promote stable donor Treg engraftment in NOD mice through immunomodulation with clinically translatable combinatorial regimens. and 2) to determine whether selection and expansion of antigen (Ag)-specific donor Tregs occurs following aCD3 and Treg infusion. This research will generate novel data that are required for the design of successful therapies based on Treg cells. Further, the demonstration of therapeutic synergy with agents that have been or are being used in clinical trials for new onset diabetes will provide impetus and needed pre-clinical data for the design of future clinical trials that combine adoptive Treg transfer with immunomodulation, and perhaps antigen-specific therapies.